This application is a national stage of PCT International Application No. PCT/EP2007/007347, filed Aug. 21, 2007, which claims priority under 35 U.S.C. §119 to German Patent Application No. 10 2006 053 563.4, filed Nov. 14, 2006, the entire disclosure of which is herein expressly incorporated by reference.
The present invention relates to a belt tensioner for a seat belt system of a vehicle, with a traction or thrust element for transmitting a tensioning force onto a seat belt strap, and with a force limiting unit for limiting the retaining force exerted on a passenger by the seat belt strap. The force limiting unit comprises a functional element coupled to the traction or thrust element, and can be displaced by means of a displaceable piston element in the effective direction of the tensioning force.
To increase the protective effect of a seat belt retention system, it is known to provide it with a belt tensioner, which withdraws the belt slack from the seat belt during an impact, so as to shorten it and/or to draw a passenger towards the backrest of the vehicle seat into a position with a reduced danger of injury. Belt tensioners, which are integrated in an automatic belt winding apparatus, and belt tensioners cooperating with a belt buckle or an end fitting of the seat belt system are known.
Belt force limiting devices are also known in connection with a seat belt system, which serve to limit the retention force acting on the passenger caught or retained by the seat belt strap to a certain extent in the case of a crash, so as to reduce the danger of an injury to the passenger by the seat belt strap.
In present-day vehicles belt tensioners are used amongst others, in which the seat belt strap is tensioned by the force limiting unit; that is, the function of the force limiting unit is superimposed over the one of the belt tensioner. In this design, the force limiting unit is released with an activated belt tensioner and when the force limiting level given by the force limiting unit is exceeded. It is thus possible that the force limiting unit already releases during the belt tensioning process, so that the seat belt strap is not tensioned to the desired extent. Such a belt tensioner provided with a force limiting unit is disclosed, for example, in German patent document DE 297 18 659 U1. It is disadvantageous with these belt tensioners, that the maximum tensioning force, which can be realized, and which is transmitted to the seat belt strap, cannot be adjusted arbitrarily, but must be considerably smaller than the release force of the force limiting unit. The reason for this is that, due to the deflection of the seat belt strap at a belt deflector in the region of about 90° to 150°, and the deflection of a traction or thrust element (which is coupled to a belt buckle or end fitting at one end, and to the belt tensioner at its other end), strong forces act in the seat belt strap in a crash, which leads to high friction losses at the deflections, as can be seen in the diagram of FIG. 9. The friction losses, which are in the region of about 20% to 40% of the tensioning force, double due to the reversal of direction (belt traction during tensioning and belt release during force limiting), which means that the maximum tensioning force can be considerably lower than the release force of the force limiting unit. The force limiting unit would otherwise always release/be activated in any case during the belt tensioning process, which is not desired.
One object of the present invention, therefore, is to provide a belt tensioner of the above-mentioned type, in which the maximum tensioning force that can be realized in the seat belt strap is larger than, equal to, or marginally smaller than the release force of the force limiting unit, without automatically releasing the force limiting unit during the tensioning process.
This and other objects and advantages are achieved by the seat belt tensioner according to the invention, which comprises a traction element that transmits a tensioning force to a seat belt strap. (The traction element may, for example, be formed of a flexible rope or a wire, or a thrust element, which can transmit traction and thrust forces.) A force limiting unit, which limits the retaining force exerted by the seat belt strap on a passenger, has a functional element coupled to a traction or thrust element, which can be displaced by means of a displaceable piston element in the effective direction of the tensioning force. The belt tensioner includes means for the parallel charge of the piston element and the traction or thrust element with the tensioning force.
Unlike the known belt tensioner, the tensioning force generated by means of a spring brake (a preferably pyrotechnical gas generator or the like) does not act solely on the piston element. Rather, it is divided into partial forces, a first of which charges the piston element in the effective direction of the tensioning force, while the second partial force simultaneously charges the traction or thrust element in the effective direction of the tensioning force. The first partial force transmitted via the piston element to the force limiting unit has only such a size that a premature release of the force limiting unit during the tensioning process by the forces caused by the belt tensioner can be virtually excluded. The second partial force, which also contributes to the belt tensioning, is not transmitted to the force limiting unit, but past it, and introduced—at least indirectly—into the traction or thrust element. The belt tensioner according to the invention has the advantage that, by the simultaneously introducing the tensioning force into the force limiting unit and the traction or thrust element, a tensioning force can be generated, which acts in the seat belt strap, and which is larger than, equal to or smaller than the release force of the force limiting unit. This means that the tensioning force and force limiting levels can be chosen/adjusted independently from each other.
In a preferred embodiment of the belt tensioner, the means for simultaneously charging the piston element and the force limiting unit with the tensioning force (or with a partial tensioning forces, which correspond to the tensioning force when added together), comprise a force transmission element, which is arranged within a working chamber on the side of the piston element turned away from the functional element. According to a first embodiment of the invention, this force transmission element can be coupled continuously, that is, permanently, to the traction or thrust element. In another embodiment, the force transmission element is coupled to the traction or thrust element only during the tensioning process, and that such coupling of the force element to the traction or thrust element is cancelled during the disabling of the force limiting unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.